1. Field of the Invention
The present invention relates to a switching circuit for use in multivoice signal demodulating circuit, and more specifically to a multivoice signal demodulating circuit for selectively outputting an audio signal from a multivoice signal including at least a main channel signal, a first subchannel signal and a second subchannel signal.
2. Description of Related Art
At present, in television broadcastings, multivoice signals are transmitted which include a main channel signal, a first subchannel signal and a second subchannel signal called "Second Audio Program" (SAP) signal. In this case, the sum of the left- and right-hand audio signals (L+R) and their difference (L-R) are ordinarily placed on the main channel signal and the subchannel signal, respectively, for a stereophonic sound tranmission and reproduction. In addition, the SAP signal is used to transmits a second language signal in a bilingual broadcasting.
In the above triple-audio broadcasting, the (L-R) signal and the SAP signal are respectively suppressed for noise reduction, for example, by a DBX noise reduction system which compresses the dynamic range at all levels and frequencies. Thereafter, subcarriers for the first and second subchannels are modulated by the suppressed signals, respectively, and further, are superposed on the main (L+R) channel signal so as to form a composite signal. In receivers, therefore, after the left- and right-hand signals are reproduced from the main (L+R) channel signal and the stereo subchannel (L-R) signal contained in the composite signal, and after the SAP signal is reproduced from the composite signal, it is necessary to expand the (L-R) signal and the SAP signal which have been suppressed for noise reduction.
In other words, the signal expansion processing is required for both the (L-R) signal and the SAP signal. On the other hand, the stereo sound and the second audio program (SAP) will not simultaneously listened. Therefore, "triple audio signal" demodulating circuits have been ordinarily constructed such that either the first subchannel signal or the second subchannel signal is selectively supplied to a DBX noise reduction decoder, and the signal restored to the original form by the decoder is fed to a stereo multplexer or outputted as the SAP signal.
However, the selective application of the signal to the noise reduction decoder has been determined by a manual switch. On the other hand, the television broadcasting is not ceaselessly in the form of stereophonic or SAP broadcasting, and the times of the SAP broadcasting are less than the stereophonic broadcasting. The stereophonic broadcasting is so made that it can be received as the monophonic broadcasting by the monophonic receivers, and the stereophonic television receivers are constructed such that the TV viewer can without interruption listen even if the stereophonic broadcasting is changed to the monophonic one.
However, when only the stereo signal is transmitted without the SAP signal, if the manual switch is operated to a SAP signal receiving position, no sound is outputted from the television receivers. Further, when both the stereo signal and the SAP signals are transmitted and the SAP signal is selected by a TV viewer, if the SAP program is terminated, a sound will suddenly disappear from the TV receiver. In this case, he has to operate the manual switch to a stereo broadcasting receiving position. Thus, in the TV receiver equipped with the conventional multivoice signal demodulating circuit, no sound is outputted from the termination of the SAP program to the operation of the manual switch. This would cause most TV viewers to be disoriented. In addition, the forced operation of the manual switch is troublesome to the TV viewers.